Sex Allocation

Question 1

Sex Ratio

Answer 1

Proportion of Males to Females in a population

Question 2

Primary Sex Ratio

Answer 2

Ratio at conception (assume male bias)

Question 3

Secondary Sex Ratio

Answer 3

Ratio at birth (male bias in humans)

Question 4

Tertiary Sex Ratio

Answer 4

Ration after puberty (female bias in humans)

Question 5

Sex Allocation

Answer 5

Investment in males vs females.
Depends on the environment, mating system, and how reproduction works in a species.

Question 6

Fisher’s Sex Allocation Theory

Answer 6

In most species, males and females are produced in approximately equal numbers.
Fisher said that the only ESS is p(da) = 0.5.
(ie. if male births are less common than females, newborn males have better prospects than females, and parents genetically disposed to producing males have more grandchildren. So, genes for male producing tendencies will spread, males become more common, and the advantage of males dies away. 1:1 is the only unbeatable strategy.)

Question 7

Departures from Equal Sex Ratios

Answer 7

Local Mate Competition
Local Resource Competition
Maternal Condition
Sex Ratio Disorders

Question 8

Local Mate Competition (LCM)

Answer 8

Selection favours a ratio biased towards the sex experiencing the least amount of kin competition. (This works in local mating systems.)
ie. Fig wasps – sons compete for access to daughters, so it makes more sense to produce more daughters than sons.

Question 9

Local Resource Competition

Answer 9

Amount of local competition can affect sex allocation depending on amount each sex competes with their parents.
ie. If sons disperse, and daughters do not, then daughters compete with their mothers for food. So there is a bias towards males.

Question 10

Maternal Condition (Trivers-Willard Hypothesis)

Answer 10

Assumptions: a) parental condition influences offspring, b) differences in offspring condition persist into adulthood, c) good condition influences the mating success of one sex more than the other.

ie. In red deer, if the mother recognizes a male is in good condition and has good territory, it pays to make a male offspring. If she recognizes he is in poor condition and has poor territory, it pays to make female offspring, as her size doesn’t depend on how much milk she gets.

Question 11

Sex Ratio Disorders (SRDs)

Answer 11

Selfish genetic elements that cause organisms to not have a 1:1 ratio.
Types: MSR, SK, PSR, Wolbachia

Question 12

SRD – MSR

Answer 12

Maternal Sex Ratio – maternally transmitted factor that causes females to fertilize all eggs, producing 100% daughter broods.

Question 13

SRD - SK

Answer 13

Son Killer – infectiously transmitted bacterium that kills unfertilized eggs (would’ve become sons)

Question 14

SRD - PSR

Answer 14

Paternal Sex Ratio – paternally transmitted factor that converts diploid fertilized eggs (would’ve been females) into haploid males that carry the PSR chromosome.

Question 15

SRD - Wolbachia

Answer 15

An inherited bacteria (present in eggs) that feminizes males, induces parthenogenesis, and produces sperm incompatibility.

Question 16

Sex Ratios in Hymenoptera

Answer 16

Determined by how many chromosomes the mother decides to give the egg. Fertilized eggs get two sets of chromosomes, and become females. Unfertilized eggs only get one set of chromosomes, and develop into haploid males.

They may sense something in the environment when making these determinations.
ie. Fig wasp manipulates sex ratio when she senses another female wasp has already laid eggs. Lays more males to compete for other females.

Question 17

Sex Ratios in Birds

Answer 17

Females with high stress hormones produce more daughters.
Low maternal condition and low food availability result in female-biased clutches.

Question 18

Sex Ratio (Theories) in Mammals

Answer 18

Maternal Testosterone Hypothesis (high testosterone – more male offspring)
Good Condition Hypothesis (higher fat levels / glucose – more male offspring)

Question 19

Seychelles Warbler

Answer 19

A bird with unusual reproductive behaviour. (Only lays one egg per clutch, nestling usually cared for by three adults, no breeding until 3yo.)

Helping by grown offspring is common. Sons usually disperse, while daughters normally stay. As such, on low quality territories, produced more sons (as it is too costly to produce daughters you compete with). On high quality territories, more daughters were produced (as it is beneficial to have a helper who stays).